The wine has started its inevitable decline.
Slow down the aging by recorking the bottle and putting it in the refrigerator. The cold will slow the chemical reaction that spoils wine. The advice goes for both white and red wines.
If the bottle is only half full, you will want to remove air from the bottle. Add marbles to the opened bottle until the level of the wine reaches the top. Recork and refrigerate.
Red wines high in tannin are the most durable. White wines are the least durable.